1. Field of the Invention
The present invention relates to an image capture apparatus that performs focus detection with a phase difference detection method using an image sensor in which image forming pixels and focus detecting pixels are arranged on an image sensing plane.
2. Description of the Related Art
Technology has been proposed for performing high-speed and high-precision focus detection using a phase-difference focus detection means in an image capture apparatus that has an imaging optical system and an image sensor. In phase-difference focus detection technology, luminous flux that exits the imaging optical system is divided into at least two areas by a pupil division means, and a pair of focus detection signal arrays of two images are obtained by photoelectrically converting the luminous flux in the areas. A so-called defocus amount, which is the amount of defocus of a predicted focal plane, is detected based the amount of relative image shift between the two signal arrays.
If the imaging optical system is in an optically in-focus state, the amount of relative image shift between the two signal arrays will ideally be zero. However, due to offset (misalignment in the optical axis direction) in the spatial arrangement of the focus detection means, optical aberration of the imaging optical system, and problems unique to the photoelectric conversion element for focus detection, there are cases where the amount of image shift between two images is not zero even when in the in-focus state. In such cases, focus detection error arises if the detected image shift amount is converted into the defocus amount as is. Accordingly, technology for resolving such error has also been proposed.
For example, Japanese Patent Laid-Open No. 58-87512 proposes the following technology. Specifically, a focus detection means is arranged at a position that is separated from the optical axis direction by a predetermined amount relative to the predicted focal plane of the imaging optical system, thus lowering the spatial frequency of the focus detection images and reducing error that arises during sampling of the photoelectric conversion element. In this case, a predetermined amount of image shift occurs between two signal arrays even in the in-focus state. In view of this, a correct focus detection result is obtained by calculating the defocus amount after subtracting the predetermined initial shift amount from the calculated image shift amount between two images.
Also, Japanese Patent Laid-Open No. 62-189415 and Japanese Patent Laid-Open No. 11-218764 disclose technology for correcting focus detection error that arises due to spherical aberration of the imaging optical system and the like. With the phase-difference focus detection methods in these two prior art documents, only a portion of luminous flux that exits the imaging optical system is used in focus detection. This results in deviation between the focal position at which imaging performance is the best and the focal position calculated based on focus detection signals. In view of this, methods for correcting the amount of such deviation are disclosed in these prior art documents.
Technology has also been proposed for using a two-dimensional CMOS sensor or the like as the imaging means and providing pixels with a phase-difference focus detection function (Japanese Patent Laid-Open No. 2001-305415). Both image forming pixels and focus detecting pixels are used in this technology. Specifically, the image forming pixels and the focus detecting pixels are arranged on the same plane, and therefore when the image forming pixels are in the in-focus state, the focus detecting pixels are also in the in-focus state. Accordingly, relative image shift does not occur in principle between two images for phase difference detection when focusing is performed. However, since pupil division is performed with very small microlenses and photoelectric conversion portions in this technology, there are cases where luminous flux is scattered in pixels and electrical crosstalk occurs between photoelectric conversion portions. Accordingly, even when in the in-focus state, there are cases where the image shift amount between two images is not zero, but rather predetermined image shift error arises. Since this image shift error arises due to individual pixel characteristics, it is difficult to resolve with the methods disclosed in the aforementioned prior art documents.
For example, since the correction of image shift between two images in Japanese Patent Laid-Open No. 58-87512 is the correction of spatial misalignment between the predicted imaging plane and the focus detection means, even if the F-number of the imaging optical system is changed, for example, there is no change in the correction amount in the defocus amount conversion.
Also, the error in Japanese Patent Laid-Open No. 62-189415 and Japanese Patent Laid-Open No. 11-218764 is caused by optical aberration of the imaging optical system. Since optical aberration normally decreases as the F-number of the imaging optical system increases, the amount of error in the defocus amount conversion decreases according to increase in the F-number.